Muscles of the lower leg and knee

Hello, everyone. This is Joao from Kenhub and welcome to another anatomy tutorial where this time we’re going to be talking about the muscles of the lower leg and knee.

So what we’re going to be doing on this tutorial is describing three groups of muscles including the anterior muscles of the lower leg, the lateral fibularis or peroneus group, and finally the posterior muscles of the lower leg.

Right now, as you can see here on this image, I'm showing you an anterior view of the lower leg and knee where you can see some of the anterior muscles of the lower leg as well as some of the lateral fibularis group.

But what I want to do in this image is strip all these muscles and expose these structures that are attached to the bones. They are vital structures that are associated to these muscles. They are origin and insertion points for muscles and also guide their tendons throughout the leg.

So before we talk about these muscles, we’re going to be talking about the structures that I'm going to list – the interosseous membrane, the inferior and superior extensor retinaculum, the inferior and superior fibular retinaculum, the flexor retinaculum of the foot, and the ligament of the head of the fibula.

We’re going to start off with this one that you see here highlighted in green from an anterior view. This is the interosseous membrane. And you can also see it here from a posterior view.

And as the name indicates “inter-osseus” membrane, this is a membrane that you find between two bones, specifically, this one here, the tibia, this thinner one, the fibula.

So this membrane is going to be running from the interosseous crest of the tibia to the interosseous crest of the fibula and serves as additional insertion point for many muscles of the leg.

Next, we’re going to be talking about these two structures that are highlighted in green which are retinacula. Specifically on the left side we’re looking at the inferior retinaculum of the foot, while on the right image, we’re looking at the superior retinaculum of the foot.

Now, retinacula are fibrous plates that fasten down the muscle tendons of the legs; so they are firmly secured and do not move around underneath the skin as you can see here on this image. These structures are almost working as belts. They fasten these tendons, so they’re just not moving around freely under your skin.

We’re going to start off with the inferior extensor retinaculum or the inferior retinaculum of the foot which has a Y-shape as you can see here also clearly on the image and runs from the calcaneus to the tibial malleolus and the medial plantar aponeurosis.

While the one, the image that you see on the right side, this is the superior extensor retinaculum or the superior retinaculum of the foot which runs from the lower end of the fibula to the lower part of the tibia.

Next, we’re going to be talking about these structures, also retinacula that you now see from a posterior view of the lower leg or a plantar view of the foot.

On the left side, we’re looking at the inferior fibular retinaculaum; while here, you see the superior fibular retinaculum.

The inferior fibular retinaculum is a lateral continuation of the inferior extensor retinaculum running towards the trochlear process of the calcaneus which you see here the calcaneus and a bit of the trochlear process of this bone.

Now, the superior fibular retinaculum runs from the lateral malleolus, which you see here this is the lateral malleolus, all the way towards the calcaneus as you can also see here on this image.

Next structure that we’re going to be seeing here highlighted in green from the same view, this is the flexor retinaculum of the foot. You can also see this image of some structures passing through the flexor retinaculum of the foot.

Now, this structure, the flexor retinaculum of the foot, will go from the medial malleolus towards the calcaneus, so the medial malleolus all the way towards the calcaneus.

Now an important note is that it forms a passage for the tibial nerve.

Last on our list, the structure that you see here highlighted in green, it’s a ligament known as ligament of the head of the fibula. You can also see here from a posterior view on the right image.

It runs from the head of the fibula towards the lateral epicondyle of the tibia. So lateral epicondyle of the tibia and the head of this thin bone, the fibula.

Now, we’re ready to start off with the very first group or two groups of muscle – the anterior and lateral muscles of the leg.

Now, before I move on and talk about the different muscles individually, I would like to list them. And before we do so, I want to tell you that the anterior and lateral muscles of the leg can be further divided into two groups. The first group is the anterior extensor group which consists of the tibialis anterior, the extensor digitorum longus muscle, the extensor hallucis longus, and the peroneus tertius.

The other group is known as the lateral fibularis or peroneus group. And keep in mind that fibularis and peroneus mean basically the same thing, so they can be used interchangeably. And you can find some schools using peroneal or peroneus and other schools using fibularis or fibular. And their names when they’re naming structures especially muscles or blood vessels which have these names on their terms.

Now, for this group, we only need to remember two muscles. And these are the fibularis longus and the fibularis brevis.

Starting with the very first one here on our list, we’re going to talk about, first, the anterior accessory group. And we’re going to talk about then the first muscle seen here highlighted in green. And yes, this is the tibialis anterior.

The tibialis anterior has origins, of course, two that you need to remember. The first one is known as the lateral surface of the tibia, as you can clearly see here on this image, which is serving as the origin point for the tibialis anterior.

And the other one is found a little bit more posteriorly known as the interosseous membrane which is found between the tibia and the fibula and will also serve as an origin point for the tibialis anterior.

And now, we move on to the insertion point of the tibialis anterior, and you can also see here a new image being introduced in this slide highlighting a bone that will serve as an insertion point for the tibialis anterior.

And this will be the plantar side of the medial cuneiform bone which will serve as insertion point for the tibialis anterior. And you can clearly notice here on this image that is originating here on the tibia and goes all the way then to insert on this bone now that is highlighted here on this image, the medial cuneiform bone, specifically on the plantar side of this bone.

Another insertion point for the tibialis anterior will be the first metatarsal bone which is this bone that you can see here on this image right here and will serve as insertion point for this muscle.

When it comes to the innervation of the tibialis anterior is going to happen. Thanks to this structure now highlighted here on the left side as you can see here highlighted in green. And this is the deep fibular nerve which is a branch of the common fibular nerve coming from the sciatic nerve and will be innervating the tibialis anterior.

Now, we’re going to move on and talk about the different functions associated to the tibialis anterior. You have probably guessed because now on the right side of the screen, you see the muscle in action and raising doing something to the foot. And this is what we’re going to talk about next.

Well, in terms of functions, you’re going to see dorsiflexion which is when you lift the back of your foot towards your tibia, and you could see represented by the arrow here on the image.

The other function that we’re going to be seeing the tibialis perform is going to be or known as supination or inversion. And remember the insertion at the plantar side of the medial cuneiform bone when I talked about the insertion points for the tibialis anterior.

Well, basically, a supination of the ankle joint is when you lift the medial side and stand on the lateral edge of you foot as you can also see represented not only by the image but also this arrow here that I just shown you.

And this is achieved because the tendon of the muscle goes below the foot and pulls up at the medial cuneiform bone.

And since we’re talking about the different functions associated to the tibialis anterior, it is important to mention that this muscle is also going to serve as the leading muscle for the supplying structures of the ankle which include the deep fibular nerve, anterior tibial artery, and the anterior tibial vein.

Now let’s move on to the next muscle on our list. And if you remember correctly, yes, this is the extensor digitorum longus.

In terms of origin points, you need to remember three. The first one is going to be the lateral tibial condyle which you can clearly see here on this image. This is the lateral tibial condyle which will serve as an origin point.

The other one is the anterior border of this bone that is hiding a little bit here behind this muscle. This is the fibula and anterior border of the fibula will be also an origin point.

The other one is the interosseous membrane that you can see a little bit here. Notice this membrane that you find between the tibia and the fibula and will serve as an origin point for the extensor digitorum longus.

You can sometimes add the head of the fibula as an additional origin point. If you want to write on your exam and do a good job, then don’t forget to also add the head of the fibula as an origin point for this muscle.

We’re going to move on to the insertion points for the extensor digitorum longus. And this muscle goes below the inferior extensor retinaculum. And it divides into four tendons inserting at the dorsal aponeurosis and also at the distal phalanges of the second through fifth toes. And you can clearly see here on this image the different tendons that will be then inserting at the distal phalanges and the dorsal aponeurosis.

Now, we’re going to move on to the innervation of this muscle. What you need to know is that just like the tibialis anterior, the extensor digitorum longus muscle is innervated by the deep fibular nerve.

And it is time for us to move on to the different functions now seen here also represented by this image on the right side. And in terms of functions, like the other extensors of the lower leg, the extensor digitorum longus is going to be performing dorsiflexion.

And if you remember correctly from the tibialis anterior, dorsiflexion is when you lift the back of your foot towards your tibia seen here represented by this arrow.

Additionally, the extensor digitorum longus is going to be performing dorsal extension of the toes from the second to the fifth toe as you can see also here on this image and is not going to be affecting the first toe here, as you can see, because it’s only inserting here on this four toes.

So the insertion points are always important when it comes to the movements that these muscles or the actions that these muscles are going to be performing. And you can clearly see here this example, so dorsal extension of the toes when you’re pulling your toes back towards the tibia.

And in opposition to the tibialis anterior that makes supination or causes supination, the extensor digitorum longus muscle forces or produces pronation seen here represented by this arrow.

Pronation or eversion is what we call; this is basically the opposite of supination as we’ve seen on the tibialis anterior. So pronation on the ankle joint is when you lift the ankle side of your foot and stand on the medial edge of the foot as you can clearly see here on this image.

So this is the lateral side, the medial side of your foot, you’re standing on the medial side and lifting the lateral side, and this is what we call pronation or eversion.

We move on, now, to the next muscle on our list seen also highlighted in green. This is known as the extensor hallucis longus. “Hallucis” is a term for big toe and “longus,” of course, for long. So you can clearly see that this is the long extensor of the big toe.

In terms of origin point, you need to remember that the extensor of the big toe has its origin at the medial side of the fibula which you can clearly see here also on this image. This is the fibula hiding here and the extensor hallucis longus is inserting on or originating on the medial side of this bone.

And also like we’ve seen on the previous muscles, the interosseous membrane also hidden here a bit, but you can notice a little portion also a little bit below here. This membrane is going to be serving as an origin point for this muscle as well.

Now, the muscle is going to go all the way from this two origin points to then insert at one point which is the distal phalanx of the big toe. And you can clearly see here distal phalanx of the big toe which will serve as the insertion point for the tendon of the extensor hallucis longus.

Next stop is going to be the innervation of this muscle. And in terms of innervation, just like the other extensors of the lower leg, the extensor hallucis longus muscle is also innervated by the deep fibular nerve.

Then we’re going to move on to the different functions of the extensor hallucis longus. And like the other extensors of the leg, it also is going to be performing dorsiflexion, as you can see here, lifting back your foot towards the tibia.

And also dorsal extension of the big toe as you can see here. So you’re going to be able to lift up your big toe towards the tibia. And it can also be involved in supination and pronation, as you can see here, of the ankle joint depending on the initial position.

Now that we covered the anterior extensor group, it is time for us to move on to the second group, the fibularis group that is now seen here represented by the two muscles that we’ve seen on that previous list, the fibularis longus and the fibularis brevis, so these are the fibularis muscles.

And these muscles are a group of two muscles at the lower leg as we’ve seen, and they lie within the peroneal compartment located at the lateral fibular region. And when lowering your foot, they can be easily seen forming the surface of the lateral lower leg.

Now, before we go on into the different details about these muscles, we’re going to just talk about briefly about their innervation as you can see here on the right image highlighted in green. This is the nerve that is going to be supplying both of the fibularis muscles. And this is the superficial fibular nerve.

Now, we’re going to start off with the very first one on the list, the fibularis longus. And you can see here that in terms of origin points, yes, a long list.

And it starts off with the lateral aspect of the fibula which will serve as an origin point, also the intermuscular septum, and the head of the fibula. So these three main origin points that you need to remember.

In terms of insertion points, you need to remember that it will insert at two bones. One is the medial cuneiform bone, and the other one is the first metatarsal bone which will serve as an insertion point for the fibularis longus.

And you can notice that it might be a bit confusing here, but if you notice that the muscle is originating here on the fibula and going all the way to then insert across the foot and goes all the way to insert on the first metatarsal bone as well and the median cuneiform bone.

Now let’s take a look at the fibularis brevis. And we need to look at the origin point for this muscle. What you need to remember here is that it is going to be originating or arising from the fibula, and you can clearly see here on this image, the muscle originating from this bone here, the fibula.

And another important note here, especially a topographical note is that the previous muscle that we talked about, the fibularis longus, is going to be covering this one, the shorter one, the fibularis brevis. So this is important to also add here.

Now, obviously, we’re going to move on and talk about the insertion point for the fibularis brevis. And what you need to know is that this muscle is going to be inserting on the tuberosity of the fifth metatarsal bone.

An important note that I would like to add here about these two muscles that we just discussed is that the tendons of both muscles run caudally towards the foot behind the lateral malleolus and then ventrally along the lateral foot edge and you can see here on both of these images.

There they are lead by two canal-like peroneal retinacula, and the inferior fibular retinaculum stretches between the lateral malleolus and also the calcaneus. The inferior fibular retinaculum stretches between the inferior extensor retinaculum of the anterior muscles of the lower leg and calcaneus.

Now, it is time for us to now look at the different functions associated to the fibularis longus and brevis. Now, the fibularis longus and brevis move both the upper and lower ankle joints.

And in the upper ankle joint, they will perform depression of the foot. So plantar flexion as their tendons run behind the flexion extension axes.

While on the lower ankle joint, their contraction leads to an eversion or pronation which means that the medial foot edge is lowered whereas the lateral foot edge will be rising.

And still on the functions here, I would like to add a note that additionally, the tendon of the fibularis longus muscle supports the transverse arch of the foot. So this is an important function here of the fibularis longus.

We are now ready to move on to the next group of muscles, the posterior muscles of the lower leg.

Now, before I go into details on each single muscle of the posterior leg, I want to divide them into groups, into layers better said.

Now, there is one superficial layer which consists of two main muscles – the triceps surae and the plantaris muscle.

The deep layer consists of muscles – the popliteus, the tibialis posterior, the flexor digitorum longus, and the flexor hallucis longus. Now these muscles are all considered part of the deep layer of the posterior leg muscles.

Now we’re going to move on to the very first muscle here on our list. But right now on your screen you have two different muscles highlighted. But these are not two different muscles; these are two muscles that are a part of this one here that we find on the first muscle on our list, the triceps surae.

Now, this is a three headed muscle that you find on the dorsal lower leg. And the heads and body of this muscle determine the surface anatomy of the calf.

Now, the triceps surae consists of the gastrocnemius as you can see here on the image which has two heads so clearly seen here, one, two heads which have eventually two origin points which we will look at. And the soleus is found underneath the gastrocnemius and has one head.

We’re going to start off with the gastrocnemius here highlighted in green. And this muscle in terms of origin points, we’re going to see that one of the heads is going to be originating from the medial epicondyle of the femur.

So as you can see here, this is the medial epicondyle and the medial head of teh gastrocnemius is originating from this portion of the bone.

You have a medial head, you would have then another one called lateral head because there is another head, the second head of the gastrocnemius is originating from the lateral epicondyle of the femur.

And as you can see clearly on this image, the gastrocnemius overlaps the soleus muscle almost completely.

Now, moving on to the insertion point of the gastrocnemius, at a distal third of the lower leg, both the soleus and the gastrocnemius muscles merge into a common tendon, famous tendon, this one right here as you can see known as the Achilles tendon which you can easily palpate on the posterior superior portion of your foot.

You can feel a tendon there, and this is the Achilles tendon formed by the gastrocnemius and also the soleus which merge at this point.

And then this tendon is going to insert in this bone right here, as you can see, known as the calcaneus. And the Achilles tendon is inserting on the posterior surface of the calcaneus at the tuberosity of the calcaneus.

So be more specific especially on your exam. Know that this is on the ... it’s inserting on the tuberosity of the calcaneus.

Now, we’re going to move on to the other muscle that we’ve been talking about, the soleus.

And we’re going to start off with the origin points. This muscle originates at the head of the fibula, the posterior border of the fibula. It is also attached to the soleal line on the tibia through a tendinous arch.

When I was talking about the insertion point of the gastrocnemius, I mentioned also the insertion point for the soleus. But it is important to remember here that these two muscles, the gastrocnemius and the soleus, will merge into the Achilles tendon and then insert at the tuberosity of the calcaneus as well.

So now that we discussed the different attachment points for the triceps surae, it is time for us to just briefly mention its innervation, meaning that both the gastrocnemius and the soleus muscle are going to be innervated by the tibial nerve which is a branch of the sciatic nerve.

Now, let’s move on to the next topic here on the triceps surae, and I'm showing you here some images; so I can talk about the different functions.

And here on the right side, you see both the gastrocnemius and the soleus contracting and for that reason producing some of the functions that we’re going to be discussing.

And on the left side you see an image of the foot. So I can show some arrows of the actions that are going to be performed by this muscle.

Its most important function is plantar flexion on the upper ankle joint as you can see here represented by these arrows which enable the lift of the heel against gravity when walking or jumping.

Plantar flexion fixes the lower leg when standing and for that reason prevents the upper body from falling forward.

In addition, it is important to add that this muscle, the triceps, is going to be responsible for or is the strongest supinator of the lower ankle joint. And supination of the ankle joint is when you lift the medial side of your foot and then you stand at the lateral edge of the foot. And you can see this movement being indicated here by these arrows.

I wouldn’t finish the functions of the triceps without mentioning that the gastrocnemius is contributing or contributes to a small extent to flexion of the knee.

We’re going to move on to the next muscle on our list, this really thin muscle known as the plantaris. And in terms of origin points for this muscle, you need to remember that it is coming from the oblique popliteal ligament and also the lateral epicondyle of the femur which you can clearly see here on this image the muscle coming from the lateral epicondyle of this bone.

Now, in terms of insertion point, you need to remember that the tuberosity of the calcaneus will serve as an insertion point via the Achilles tendon.

If you look at the innervation of the plantaris, all you need to remember like the previous ones, the previous muscle that we talked about. The plantaris is going to be innervated by the tibial nerve.

Now, the deep posterior muscles of the lower leg are part of the calf musculature, and we’re going to be talking about them right now.

They are located within the deep posterior compartment and include these that you’re seeing right now, the popliteus, tibialis posterior, the flexor digitorum longus, and the flexor hallucis longus.

And before we do so, I want to do a clarification here that the deep posterior muscles are supplied in terms of innervation by the tibial nerve.

So the tibilias posterior, the flexor digitorum longus, and the flexor hallucis longus, the popliteus muscle are going to be innervated by this nerve that you can see here on the image highlighted in green, the tibial nerve.

We’re going to start off with the very first muscle of the deep layer here seen highlighted in green, the popliteus. And in terms of general information that you need to know, it’s a small muscle located at the knee joint. And sometimes we consider it as a part of the posterior thigh musculature.

If you look at the origin points for this muscle, it originates at the lateral condyle of the femur as you can see here. And also the lateral meniscus of that you find on the knee.

If we’re going to look at the insertion point for this muscle, you need to remember that after running medial caudally towards the tibia, it inserts above the origin of the soleus muscle at the posterior surface of the tibia, and you can clearly see also here on this image.

As I mentioned before, but just as a reminder in terms of innervation, this muscle is going to be innervated by the tibial nerve.

We’re going to move on and talk about the different functions of the popliteus. And a general task for this muscle is to stabilize the dorsal knee region. And also is going to be causing internal rotation as you can see here.

It is responsible for reversing the so called terminal rotation. This refers to the locking of the tibia and femur during knee extension by slight external rotation about five degrees.

And even though the popliteus muscle anatomically ranks amongst the flexors of the thigh musculature, its ability to flex the knee is truly negligible. So it’s important to add here on the functions of the popliteus.

We are going to move on to another muscle here that we need to discuss on the deep layer. This is the tibialis posterior.

And in terms of origin point, it is going to originate from the interosseus membrane of the leg, and also the posterior surface of the tibia, and the posterior surface of the fibula.

And you can clearly see here by the location of the muscle that is going to be inserting both on the fibula that you can see here and the tibia and, of course, the membrane that stands between this two bones.

If we look at the different insertion points for the tibialis posterior, it inserts at the medial border of the foot at the tuberosity of the navicular bone and the cuneiform block bones which includes the medial cuneiform, the lateral cuneiform, and the intermediate cuneiform.

So remember this, all the cuneiform bones are going to serve as an insertion point for the tibialis posterior. And you can clearly see here that it’s quite expanding to different bones. The insertion point is expanding or the tendon is expanding to different bones here. And distally, it forms the tendon sheet extending up to the lateral foot border.

We’re going to move on and talk about the different functions associated to the tibialis posterior. It is involved in plantar flexion of the upper ankle joint, another function associated to the tibialis posterior is going to be supination in the lower ankle joint represented here by this arrow which basically is when you lift the medial side of your foot while standing on the lateral side or using the lateral side of the foot as support.

Now also to add here in the functions is that this muscle has what is known to be a tendon sheet which will support the transverse and longitudinal arches of your foot.

Now let’s move on to the next muscle seen here highlighted in green. This is known as the flexor digitorum longus.

In terms of origin points, it is easy. You need to remember that it is coming or this muscle arises on the posterior surface of the tibia. And you can clearly see here located on the medial side.

And for that reason, you can notice that this bone here, notice a little bit of the tibia here and a bit more here. This is going for the posterior surface of the tibia in serving as an origin point.

Now if you look at the insertion point for this muscle, it curves laterally around the navicular bone. As you notice here on this image clearly.

And there, the tendon fans out into four smaller tendons which insert at the bases of distal phalanges of the second to fifth toes.

Now, let’s take a look at the different functions associated to the flexor digitorum longus. And what you need to know about this is that this muscle is involved in plantar flexion in the upper ankle joint as seen here indicated by these arrows.

And another thing that is important to add here is that it’s going to be also able to cause supination in the lower ankle joint, or inversion. You get it here by this arrow. Just basically when you lift the medial side of your foot, and you stand on the lateral side or use the lateral side of the foot as support.

Now, the other function that is important to mention here and that you can clearly see illustrated here in this image is flexion of the toes and seen here the arrow indicating the movement.

And as I mentioned on the previous slides about the different tendons here of the flexor digitorum longus inserting on the different toes, this will enable to pull when the muscle is contracting. It’s going to pull the toes down or towards the plantar side of the foot, and this is what we call flexion of the toes.

Now we’re going to move on to another muscle on our list. This one seen also here highlighted in green. And this is the flexor hallucis longus.

Like the name indicates, this is the flexor. This muscle is going to be flexing the big toe. “Hallucis,” Latin for big toe and is the long flexor of the big toe.

And in terms of origin points, you need to remember that this muscle is going to come from the posterior surface of the fibula as you can clearly see on this image, so located a bit more laterally leading it to then originate from this bone here as you can see here on the image, the fibula.

Now another part that is or another structure that is going to serve as origin point is going to be the interosseus membrane of the leg.

If you look at the insertion points for the flexor hallucis longus, it runs under the sustentaculum tali of the calcaneus and finally inserts at the base of the first distal phalanx or the base of the distal phalanx of the big toe. And you can clearly also see here on this image, so it’s inserting on the base of the distal phalanx of the toe.

Now this muscle forms a tendon sheet at the foot sole as well.

Now, if we’re looking at the different functions of the flexor hallucis longus, like all deep posterior muscles, it is of the leg. It is involved in plantar flexion of the upper ankle joint and also supination or inversion of the lower ankle joint seen here indicated by this arrow as well.

Now, we’re still on the different functions associated to the flexor hallucis longus. And we have this image here to illustrate the next one. And this one is called the flexion of the big toe.

So this muscle is going to be able to flex your big toe. So when you pull your big toe down, this muscle is going to be definitely involved in this movement.

Now what happens is that if you’re flexing your big toe, usually you may flex a bit the second and third toe. And this happens due to the fact, if you notice here on this image clearly illustrates here crossing of the tendon of the flexor hallucis longus and also the flexor digitorum longus that we talked about. They’re crossing.

So once this one is contracting, the flexor hallucis longus, this tendon may also affect the nearby tendons and cause flexion of the second and third toe. This place is what we call the plantar chiasm.

Furthermore, the tendon sheet of the flexor hallucis longus supports the transverse and longitudinal arches of your foot.